Abaixo está uma instrução que descreve uma tarefa de geração de SQL, combinada com uma entrada que fornece mais contexto sobre os esquemas de tabela disponíveis. Escreva o código SQL que responda adequadamente à solicitação
 ###Instrução: questão: quantos vencedores do Vietnã
 ###Tabelas: CREATE TABLE averages (averages_id AUTO_INCREMENT PRIMARY KEY, Rank by average INT, Place INT, Couple VARCHAR, Total points INT, Number of dances INT, Average INT) CREATE TABLE performances_by_nations (performances_by_nations_id AUTO_INCREMENT PRIMARY KEY, # INT, Nation VARCHAR, Winners INT, Runners-up INT, 3rd Place INT, 4th Place INT) CREATE TABLE station_repeaters (station_repeaters_id AUTO_INCREMENT PRIMARY KEY, Callsign VARCHAR, Network VARCHAR, City of License VARCHAR, D ERP VARCHAR, Owner / Operator VARCHAR) CREATE TABLE 2012_events (2012_events_id AUTO_INCREMENT PRIMARY KEY, Date VARCHAR, Event # VARCHAR, Event VARCHAR, Winner VARCHAR, Prize VARCHAR, Entries VARCHAR, Prize Pool VARCHAR, Elapsed Time VARCHAR) CREATE TABLE 1818-1821_orders (1818-1821_orders_id AUTO_INCREMENT PRIMARY KEY, Name VARCHAR, Built by HM Dockyard VARCHAR, Ordered VARCHAR, Laid down VARCHAR, Launched VARCHAR)  
SELECT Winners FROM performances_by_nations WHERE Nation = Vietnam